The present invention relates to a light-weight, high-hardness and good-moldability polyurethane foam which can be used for, for example, sun visors or hood silencers, which are motor vehicle parts, and to a method for manufacturing the same.
In order to improve the fuel consumption or the like, the weight of a car is desired to be reduced. Polyurethane foam used for vehicle parts such as sun visors a hood silencer is also expected to have a lower density while maintaining the physical properties. Hood silencers, which is motor vehicle parts, or the like are formed from a cover material and a polyurethane foam to have a desired three-dimensional shape by hot press molding, in which the cover material and the polyurethane foam are laminated and bonded together in a mold. Conventionally, it has been difficult to reduce the apparent density of polyurethane foam to 30 kg/m3 or less. Specifically, in order to manufacture polyurethane foam having an open cell structure, a predetermined hardness, and a low density, it is necessary to increase the content of water as a blowing agent in a raw material of the polyurethane foam. In this case, the increase in the content of water causes an increase in the exothermic temperature of the raw material to 170° C. or higher. As a result, there is a possibility of self-ignition based on oxidation deterioration (scorching) of polyurethane, and the resulting polyurethane foam is discolored according to the scorching. In order to avoid such a situation, it is known that a blowing auxiliary is added to the raw material with the conventional content of water. As a blowing auxiliary, methylene chloride and liquefied carbon dioxide may be used.
However, methylene chloride is one of the substances that have an adverse influence on environment, and its use is regulated. On the other hand, foaming with liquefied carbon dioxide requires equipment exclusive for supplying liquefied carbon dioxide at a high pressure, and in order to perform foaming smoothly, not only manufacturing conditions are limited but also manufacturing costs are increased. Japanese Laid-Open Patent Publication No. 6-199973 discloses a technique where a polyolefin powder, such as polyethylene powder, is added to a polyurethane foam raw material for the purpose of causing endothermic reaction. Japanese Laid-Open Patent Publication No. 6-41266 discloses another technique where a styrene-containing polymeric polyol and an amino alcohol are used as polyol, which is a raw material for a polyurethane foam. The polyurethane foam obtained by this technique can be laminated with a cover material by hot press molding at a low temperature and then formed into a desired shape by vacuum forming.
However, for the technique described in Japanese Laid-Open Patent Publication No. 6-199973, though the effect is observed of lowering the exothermic temperature during the reaction and foaming of the raw material, the blending quantity of the polyolefin powder needs to be increased so as to effectively suppress the amount of the exothermic heat. In that case, the density of the resultant flexible polyurethane foam becomes excessively high and the physical properties such as residual compression strain deteriorate due to the increased amount of polyolefin powder. To prevent the deterioration of such physical properties, the blending quantity of the polyolefin powder needs to be reduced. Thus, the technique described in Japanese Laid-Open Patent Publication No. 6-199973 cannot effectively lower the exothermic temperature during the reaction and foaming of the raw material, leading to the problem of being unable to suppress the discoloration due to scorching.
The technique described in Japanese Laid-Open Patent Publication No. 6-41266 simply uses a polymeric polyol and an amino alcohol together as polyols. Japanese Laid-Open Patent Publication No. 6-41266 states that the hot-press moldability of the resultant polyurethane foam is good at low temperatures. However, the density of the polyurethane foams of Examples 1 and 2 described in the patent document was as high as 51 to 52 kg/m3. The technique described in Japanese Laid-Open Patent Publication No. 6-41266 poses the problem of being unable to inhibit the discoloration of the polyurethane foam because the exothermic temperature increases when trying to lower the density of the polyurethane foam.